Iran is experiencing surprising setbacks in its efforts to enrich uranium, according to new assessments that suggest that equipment failures and other difficulties could undermine that nation's plans for dramatically scaling up its nuclear program.

Former U.S. officials and independent nuclear experts say continued technical problems could also delay -- though probably not halt -- Iran's march toward achieving nuclear-weapons capability, giving the United States and its allies more time to press for a diplomatic solution. In recent months, Israeli officials have been less vocal in their demands that Western nations curtail Iran's nuclear program.

Indications of Iran's diminished capacity to enrich uranium arise just as the Obama administration begins to take sterner action to compel Iran to abandon enrichment. On Wednesday, the Treasury Department announced new U.S. sanctions against companies it says are affiliated with Iran's Revolutionary Guard Corps, a key player in the country's nuclear and missile programs.

While Iran says its nuclear program is entirely peaceful, Western nations suspect that the country is intent on developing an atomic bomb. The program prompts frequent international posturing, such as Iran's announcement last year that it would expand its nuclear facilities tenfold and more recent statements from Western leaders that the time has come to apply tougher international sanctions against the country.

Beneath this rhetoric, U.N. reports over the last year have shown a drop in production at Iran's main uranium enrichment plant, near the city of Natanz. Now a new assessment, based on three years of internal data from U.N. nuclear inspections, suggests that Iran's mechanical woes are deeper than previously known. At least through the end of 2009, the Natanz plant appears to have performed so poorly that sabotage cannot be ruled out as an explanation, according to a draft study by David Albright, president of the Institute for Science and International Security (ISIS). A copy of the report was provided to The Washington Post.

The ISIS study showed that more than half of the Natanz plant's 8,700 uranium-enriching machines, called centrifuges, were idle at the end of last year and that the number of working machines had steadily dropped -- from 5,000 in May to just over 3,900 in November. Moreover, output from the nominally functioning machines was about half of what was expected, said the report, drawing from data gathered by the International Atomic Energy Agency, the U.N. nuclear watchdog.

A separate, forthcoming analysis by the Federation of American Scientists also describes Iran's flagging performance and suggests that continued failures may increase Iran's appetite for a deal with the West. Ivan Oelrich, vice president of the federation's Strategic Security Program, said Iranian leaders appear to have raced into large-scale uranium production for political reasons.

"They are really struggling to reproduce what is literally half-century-old European technology and doing a really bad job of it," Oelrich said.

The findings are in line with assessments by numerous former U.S. and European officials and weapons analysts who say that Iran's centrifuges appear to be breaking down at a faster rate than expected, even after factoring in the notoriously unreliable, 1970s-vintage model the Iranians are using. According to several of the officials, the problems have prompted new thinking about the urgency of the Iranian nuclear threat, although the country has demonstrated a growing technical prowess, such as its expanding missile program.

"Whether Iran has deliberately slowed down or been forced to, either way that stretches out the time," said Patrick Clawson, deputy director for research at the Washington Institute for Near East Policy, a nonpartisan think tank.

But analysts also warned that Iran remains capable of making enough enriched uranium for a small arsenal of nuclear weapons, if it decides to do so. Iran has announced plans to build 10 new uranium plants, and on Monday the government said it would begin increasing the enrichment level of some of its uranium, from a current maximum of 3.5 percent to 20 percent. Enrichment of 90 percent is considered weapons-grade.

Some officials suggested that the apparent drop in output could be a ruse, an attempt by Iran to disguise its true capability until it is ready to test a nuclear device. Iran acknowledged last year that it had built a secret enrichment facility inside a mountain bunker near the ancient city of Qom, leading to suspicions that there could be other hidden sites.

"The IAEA measurements at Natanz are very crude and easily subject to intentional manipulation," said a former U.S. official who has closely monitored Iran's nuclear progress. He predicted that the watchdog agency eventually "will see that Iran is hiding production and is underreporting their success."

The administration's announcement of new sanctions represents stepped-up enforcement of existing punitive measures against Iran as the White House prepares to push for concerted action by the U.N. Security Council, the European Union and a coalition of major trading partners in an effort to force Iran to address international concerns over its nuclear ambitions.

U.S. officials are considering additional sanctions against companies linked to the Revolutionary Guard Corps, as well as finance, insurance and other entities connected to the government elite, but Russian and Chinese acquiescence is not guaranteed.

Russia has warned that it probably would not support economic measures, although Russian Deputy Foreign Minister Sergei Ryabkov told the Interfax news agency Wednesday that Iran's decision this week to begin producing higher-enriched uranium has given "additional relevance" to a new sanctions resolution. China has remained cool to new sanctions.

This week's action freezes the assets of four companies that Treasury said are owned or controlled by, or act on behalf of, a major contractor known as Khatam al-Anbiya, which has channeled billions of dollars a year to the Guard from its activities in oil, construction, transportation and other industries. The action also targets Guard Gen. Rostam Qasemi, who is the commander of Khatam al-Anbiya Construction Headquarters.

The Guard has received at least $6 billion worth of government contracts in two years, according to state-run media, but the total is likely much higher because many contracts are not disclosed. Working through its private-sector arm, the group operates Tehran's international airport, manages Iran's weapons manufacturing business and is involved in other industries.

"Today's action exposing Khatam al-Anbiya subsidiaries will help firms worldwide avoid business that ultimately benefits the IRGC and its dangerous activities," said Treasury Undersecretary Stuart A. Levey. The subsidiaries are: the Fater Engineering Institute, the Imensazen Consultant Engineers Institute, the Makin Institute and the Rahab Institute.

Also Wednesday, Iran rejected an offer from the United States to help provide it with a steady supply of medical isotopes, meaning that it will stay with its plan to produce uranium enriched to 20 percent in order to feed an aging U.S.-built research reactor that can make the isotopes.

Iranian officials have told the IAEA that the country will produce its first batch of higher-enriched uranium within a few days, but the officials also disclosed that the effort will be modest, involving a small amount of uranium feedstock and a fraction of Iran's capacities, according to a confidential U.N. document obtained by the Associated Press.

The enrichment program's troubles have been documented by IAEA inspectors who have visited the Natanz plant at scheduled intervals to collect samples and take measurements to ensure that Iran isn't diverting the enriched fuel for a clandestine weapons program.

As the ISIS study notes, the Natanz plant initially exceeded expectations, producing steadily higher amounts of low-enriched fuel. But sometime in late 2008 or early 2009, the output dropped from about 90 to 70 kilograms per month. Overall production improved slightly after that but struggled to return to 2008 levels, even as Iranian scientists installed more centrifuges, the report said. In late 2009, the 3,900 machines listed as functional were generating half the amount of enriched uranium expected, it said.

Neither Iran nor the U.N. watchdog have officially accounted for the slumping output, and U.S. officials have declined to speculate publicly about the reasons. The ISIS report identifies the likely cause as a combination of poor design and Iran's rush to put complex assemblages of centrifuges into production before working out the bugs. The report cites "daily attrition through breakage," as well as a failure to anticipate the difficulty of operating large numbers of machines simultaneously.

"Iran has moved too quickly to install centrifuges, at the expense of developing competence in operating them reliably," said the report, co-authored by Albright and Christina Walrond. "In the process it has made many mistakes."

Also, while there is no hard evidence pointing to sabotage, ISIS acknowledges the possibility that Natanz's problems were caused by outside sources. "It is well known that the United States and European intelligence agencies seek to place defective or bugged equipment into Iran's smuggling networks," it said.